Recently, in order to satisfy conflicting requirements of improving crash safety of automobiles and decreasing the weight of automobile bodies, a steel sheet used in automobile bodies is required to have both higher strength and higher ductility. As a high-strength steel sheet that may satisfy these requirements, the following have been disclosed. A production method for high-strength steel sheets is disclosed in Japanese Unexamined Patent Application Publication No. 62-182225, and in this production method, hot-rolling, cold-rolling, and annealing are performed on a steel including 0.1 to 0.45% of C and 0.5 to 1.8% of Si under predetermined conditions. In this case, a steel sheet having a tensile strength of 82 to 113 kgf/mm2 and a good ductility in which the tensile strength multiplied by elongation is not less than 2500 kgf/mm2% is produced.
Another production method for a high-strength steel sheet is disclosed in Japanese Unexamined Patent Application Publication No. 7-188834, and in this production method, a steel including 0.1 to 0.4% of C and limited Si is adjusted to have a larger amount of Mn and is annealed twice under predetermined conditions. In this case, a steel sheet having a tensile strength of 811 to 1240 MPa and a high ductility in which the tensile strength multiplied by elongation is not less than 28000 MPa·% is produced.
Another production method for a high-ductility and high-strength cold-rolled steel sheet is disclosed in Japanese Unexamined Patent Application Publication No. 61-3843, and in this production method, an austenite volume fraction of a steel including 0.02 to 0.3% of C during annealing is adjusted to be in a predetermined range. In this case, a steel sheet in which a tensile strength is 48 to 151 kgf/mm2 and the tensile strength multiplied by elongation is not less than 1800 kgf/mm2% is produced.
Since formability of a steel sheet is decreased when strength of the steel sheet is increased, use of a high-strength steel sheet is limited to parts having a simple shape. Accordingly, in order to overcome this drawback, attempts were made to use steel sheets having large difference between static and dynamic strengths.
In general, deformation strength of a steel sheet is affected by a strain rate, and deforming stress is increased as the strain rate increases. That is, if the difference between static and dynamic strengths is large, formability of the steel sheet is ensured because the strength of the steel sheet is relatively low in press forming, whereas a sufficient strength is ensured during rapid deformation in automobile collisions.
For example, a thin steel sheet having a superior impact resistance and a production method therefor are disclosed in Japanese Unexamined Patent Application Publication No. 7-3381. In this case, in a ferrite single phase steel, amounts of solid-solved C and solid-solved N in the ferrite are decreased, and the amount of cementite is appropriately adjusted, so that the difference between static and dynamic strengths of the steel sheet is increased. A production method for high-strength steel sheets having an ultrafine structure is disclosed in Japanese Unexamined Patent Application Publication No. 2000-73152. In this case, plural metal sheets, the surfaces of which had been cleaned, are laminated, and accumulative roll-bonding is performed on the metal sheets so that ferrite grains are refined and are of sizes on the order of nanometers, that is, smaller than 1 μm. A high-strength and high-ductility steel sheet and a production method therefor are disclosed in Japanese Unexamined Patent Application Publication No. 2002-285278. In this case, the steel sheet is formed by cold rolling and annealing a martensite of a normal low carbon steel, so as to generate an ultrafine ferrite and cementite structure which have a superior balance of strength and ductility.
As described above, the principal purpose of applying a high-strength steel sheet to automobile bodies is to decrease impacts on occupants by effectively absorbing the impact energy of crashes, and the high-strength steel sheet is desirably used for more of the parts. However, the following two problems occur in trying to achieve this.
First, since the ductility of a steel sheet decreases as the strength of the steel sheet increases, press formability is decreased, and the use of the steel sheet may be limited to parts having a simple shape. Second, a member made of the steel sheet may be fractured during a crash. That is, when a head-on crash occurs, which is a major type of automobile crash, parts such as a front frame absorb the impact energy by receiving the loads in longitudinal directions thereof and buckling. At that time, if the ductility of the steel sheet is low, the material is fractured in the crash deformation, and the impact energy may not be efficiently absorbed. Therefore, a steel sheet having higher tensile strength and higher ductility compared to those of conventional steels is required. Next, problems of high-strength steel sheets produced by conventional techniques will be described.
A steel sheet having a complex structure of ferrite and residual austenite is disclosed in Japanese Unexamined Patent Application Publication No. 62-182225, and the steel sheet has a superior balance of strength and ductility, but requires addition of not less than a certain amount of Si, which deteriorates surface characteristics. In addition, 0.36% of C is required in order to obtain a steel sheet having a high strength of 1000 MPa or more, whereby the strength of spot welding is small, and tensile strength of the steel sheet will be approximately 113 kgf/mm2 at most.
A production method for a high-strength steel sheet having a good balance of strength and ductility with less Si is disclosed in Japanese Unexamined Patent Application Publication No. 7-188834. In this method, the production cost is high because annealing must be performed twice, strength of spot welding is small because the amount of C is large, and tensile strength of the steel sheet is not more than 1240 MPa. A production method for a high-strength steel sheet having tensile strength of 1300 MPa or more is disclosed in Japanese Unexamined Patent Application Publication No. 61-3843. In this case, a steel sheet having tensile strength of 1500 MPa exhibits elongation that is not more than 12% and does not have a good balance of strength and ductility.
In the above conventional techniques, alloying elements are added to a steel, and the steel is heat treated so as to have a complex structure of a ferrite phase and a hard second phase such as martensite, bainite, and residual austenite, thereby increasing strength of the steel. In this case, a high-strength steel sheet having a tensile strength of 1300 MPa or more and a ductility of more than 12% cannot be obtained. Substantial amounts of alloying elements such as C, Si, and Mn are required in order to obtain a steel sheet having a tensile strength of 800 to 1300 MPa, whereby the strength of spot welding of the steel sheet is low, and the production cost and recyclability of the steel sheet are inferior. Therefore, a technique for producing steel sheets including minimized amounts of such alloying elements and having a good balance of strength and ductility is required.
The present inventors have focused on a refinement of ferrite grains as a method for strengthening steels, which is not based only on conventional methods as described above. That is, in this method, a steel sheet is strengthened by increasing the area of grain boundaries of a ferrite phase in a matrix, addition amounts of alloying elements are minimized, and the purity of the ferrite is maintained at a high level. This method is based on the idea that the difference between static and dynamic strengths of a steel sheet can be obtained by maintaining the purity of a ferrite phase so that it is as high as possible.
The relationship between the grain size and the strength is known from the Hall-Petch equation, and the deformation strength is proportional to the −½ the power of the grain size. According to the equation, since the strength is considerably increased when the grain size is less than 1 μm, grains of a steel sheet must be refined to be ultrafine grains having a size of not more than 1 μm so as to extremely increase the strength of the steel sheet.
The technique of appropriately adjusting the amount of cementite, which is disclosed in Japanese Unexamined Patent Application Publication No. 7-3381, is based on the idea that the difference between static and dynamic strengths can be improved by minimizing the amounts of impurity elements in a ferrite. Tensile strengths of steel sheets obtained by this method are approximately 430 MPa, which is insufficient for use as a high-strength steel sheet. Since the tensile strength of a steel sheet having a structure of a ferrite single phase cannot be greatly increased, in order to obtain a higher-strength steel sheet, the structure is generally formed so as to be a complex structure of a ferrite and a second phase such as martensite. If a steel sheet is formed so as to have a complex structure, the tensile strength thereof is improved, but the difference between static and dynamic strengths is decreased, and it is smaller than that of a steel with a ferrite single phase, which is typical as a soft steel sheet. For example, the following fact is disclosed on page 174 of “Report of Workshop on Rapid Deformation of Automobile Materials” compiled by The Iron and Steel Institute of Japan (2001). Whereas a soft steel exhibits a difference between static and dynamic strengths of 210 MPa at 5% strain, a dual phase steel which can exhibit a difference between static and dynamic strengths of 590 MPa exhibits a difference between static and dynamic strengths that is decreased to approximately 60 MPa at 5% strain. On the other hand, ductility in rapid deformation, and specifically, a uniform elongation of the dual phase steel which can exhibit the difference between static and dynamic strengths of 590 MPa are greater than those of the soft steel. Steels having high ductility at rapid deformation, such as the dual phase steel, are desirable in view of avoiding rupture of automobile parts during crashes.
Therefore, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 7-3381, it is difficult to achieve both a large difference between static and dynamic strengths and a high tensile strength simultaneously, and to achieve both a large difference between static and dynamic strengths and a high ductility at rapid deformation simultaneously. Accordingly, a high-strength steel sheet having a large difference between static and dynamic strengths, high tensile strength, and high ductility at rapid deformation is desired.
Since the method of rapid deformation testing has not yet been standardized, a method such as the Hopkinson bar method, the one-bar method, and the sensing-block method have each been used, and the shape of the test specimen differs according to the method. Therefore, the yield point and the overall elongation may be different according to the method, and stress-strain diagrams obtained using different fast tensile test methods are not comparable. Moreover, a test result obtained by using a test specimen having a shape of No. 5 specified by the Japanese Industrial Standard (JIS), which is generally used in a quasi-static tensile test, and a test result obtained by using a smaller test specimen which is generally used in a fast tensile test, may be different even when the tests are performed at the same strain rate. Accordingly, test results obtained by using the same test apparatus and test specimens having the same shape and by varying only stain rate should be compared, or else the test results cannot be accurately compared. In the following descriptions relating to characteristics such as deforming stress and elongation, data of the characteristics measured by the following method will be used. That is, measurements were performed by using a sensing block type high speed material testing machine manufactured by Saginomiya Seisakusyo, Inc. and using test specimens with a shape shown in FIG. 14, and only the strain rate was varied.
The above Japanese Unexamined Patent Application Publication No. 2000-73152 may be mentioned as an example of a method of refining the grain sizes of ferrite of a steel sheet to the order of nanometers, that is, smaller than 1 μm. In this method, by repeatedly performing accumulative roll-bonding for 7 cycles, the structure becomes an ultrafine structure having grain sizes on the order of nanometers, and the tensile strength reaches 3.1 times (870 MPa) as high as that of the IF steel which was used as the raw material. However, the method has two drawbacks.
The first drawback is that a material having a structure consisting only of ultrafine grains of which the sizes are not more than 1 μm has extremely low ductility. The reason for this is described in a paper written by the inventors of Japanese Unexamined Patent Application Publication No. 2000-73152, for example, “Iron and Steel” (The Iron and Steel Institute of Japan, Vol. 88 (2002), No. 7, p. 365, FIG. 6b). According to this paper, when the grain sizes of ferrite are less than 1.2 μm, the overall elongation is suddenly decreased, and the uniform elongation is simultaneously decreased to approximately 0. Such a structure is not suitable for steel sheets to be press formed.
The second drawback is that performing the accumulative roll-bonding repeatedly in an industrial process decreases the production efficiency and greatly increases the production cost. Large strain is required for ultra-refining the grains, and for example, the grains are not ultrarefined until 97% of the strain in terms of rolling reduction is applied by performing the accumulative roll-bonding for 5 cycles. This ultra-refinement cannot be practically performed by an ordinary cold rolling in which the production efficiency is good, because the steel sheet needs to be rolled from a thickness of 32 mm to a thickness of 1 mm.
In Japanese Unexamined Patent Application Publication No. 2002-285278, by cold rolling and annealing martensite as an initial structure, the martensite forms an ultrafine ferrite and cementite structure having a good balance of strength and ductility, whereby the balance of strength and ductility of a steel sheet is improved. An invention example disclosed in Japanese Unexamined Patent Application Publication No. 2002-285278 has advantages in that the amount of C is 0.13% and is relatively small, and that addition amounts of alloying elements are small. However, the example exhibits a tensile strength of 870 MPa and an elongation of 21%, and therefore, the example does not have satisfactory characteristics.